I know it might be bit late to reply, but there are three different places where you can level up easily and fast. I hope that it will be helpful. So,
First place is early in game (Kingdom of Corona), just keep moving between two save points and you will earn more than 2.000 exp, which is a lot for your characters in early game.
Second place is in mid game in (Monstropolis), there is an in-game spot where you have to fill laugh bar and even after filling it up, the heartless will keep on coming unless you press "triangle" for scream reaction command. So instead of doing that, just keep fighting heartless as long as you want.
Third place is after completing game in (San Fransokyo), there is a portal that gives you 500 Munny and 30.000 exp per minute. So keep playing it to level up fast and also to unlock "Leveled Out" trophy.